errno with FFI

[keturn]

An experience report from using FFI to a libc function:

Deno's filesystem API doesn't have mkfifo, so I went to see if I could invoke it via FFI. I found the Deno.dlopen interface straightforward to use, especially for this case where the API I'm interested in is very small and stable.

The happy path worked great.

Next I went to check on the error handling. libc error handling is as cumbersome as it ever was, but I went through the motions: check the static global variable errno, consult strerror() to get something human-readable, etc… It was a bit of a chore, as surely the runtime must already have implemented this somewhere before, but I couldn't find it exposed for our use.

Eventually I got it to produce an error message saying “No such file or directory [ENOENT]”. A significant improvement over “Error.”

I called my function again with different arguments to trigger a different type of error case, and— “No such file or directory” again.

🫤 … no, that's not the appropriate error message this time. What went wrong?

const libc = Deno.dlopen("libc.so.6", {
    mkfifo: { parameters: ["buffer", "i32"], result: "i32" },
    errno: { type: "i32" },
});

The thing I didn't realize is that though libc.symbols.errno was looking at the errno I intended it to, the properties of the DynamicLibrary.symbols are only set once, when it's created. It's not a live view. No matter how many errors I cause, that value is stuck at whatever it was when I first called dlopen.

Deno.dlopen is capable of doing what we need here, it just took a little more rummaging with unstable interfaces with "Unsafe" in their name:

const libc = Deno.dlopen("libc.so.6", {
    errno: { type: "pointer" },
});
const errnoPtr = new Deno.UnsafePointerView(libc.symbols.errno);
// …
errno = errnoPtr.getInt32();

[bartlomieju]

@aapoalas can you take a look and advise?